Four people in a Northwest Washington apartment were hospitalized early yesterday after suffering carbon monoxide poisoning, apparently from a leak in a defective boiler, according to fire officials.

The residents, including a 70-year-old woman, a 3-year-old boy and a man and a woman in their twenties, were in a second-floor apartment in the 7300 block of Georgia Avenue NW when the elderly woman called 911 about 8 a.m., according to fire department spokesman Alan Etter. Two adults were found to have suffered the most exposure, Etter said.

All four residents were taken to Washington Hospital Center for treatment. By early afternoon, all but one had been released, according to a hospital worker.

No other people were in the building's three other apartments or in the stores on the first floor, Etter said.

Carbon monoxide is an odorless, colorless gas that at high levels can kill in minutes. It is produced whenever any fuel, such as gasoline, oil, kerosene, wood or charcoal is burned, according to the federal Environmental Protection Agency. If appliances are not working properly or are used incorrectly, dangerous levels of carbon monoxide can result.

According to the EPA, hundreds of people die accidentally every year from carbon monoxide poisoning caused by malfunctioning or improperly used fuel-burning appliances.

"The only time you know you're suffering is when you start to throw up, have headaches and feel flushed," Etter said.

There were no carbon monoxide detectors in the building, Etter said. A hazardous materials team found levels of the gas to be more than 200 parts per million in the building, about 40 times higher than the acceptable level, according to Etter.

Firefighters Steven Kelly, 47, and Mark Murphy, 28, were the two rescue workers in the Engine Company 22 ambulance that initially responded to the call. Kelly said they were at the scene for about 40 minutes and were also exposed to the gas. Both felt lightheaded, he said, and were also treated and released.

Kelly said the first call was for a sick person. They initially treated the younger woman and the man. But when the elderly woman started having convulsions, he said they suspected carbon monoxide poisoning and called for additional medical units.

City officials contacted the building's owners, and a contractor was sent to repair the boiler, Etter said. Property records show the owners as living in Silver Spring, but no one answered the telephone yesterday at that address.